1. Field of the Invention
The present invention relates to an ink jet printing apparatus and method that carries out printing using a connecting head in which a plurality of short chips are arranged in a given direction to form a long head, each of the chips having an array of a plurality of nozzles through which ink is ejected.
2. Description of the Related Art
Printing apparatuses are now used as printers, printers used in copiers or the like, composite electronic apparatuses including computers and word processors, or output apparatuses for workstations. These printing apparatuses are configured to print images (including letters and symbols) on print media such as paper and plastic thin plates on the basis of print information. Various printing apparatuses have been proposed which are based on respective printing strategies. For example, printing strategies such as an ink jet strategy, a wire dot strategy, and a thermal strategy are known to use a print head to form dots on print media on the basis of print information. A known printing strategy using no print heads is a laser beam strategy that irradiates a photosensitive drum with a laser beam on the basis of print information.
The printing strategy using a print head is commonly used owing to the small size and low cost of the corresponding apparatus. A serial type printing apparatus adopts this printing strategy. The serial type printing apparatus carries out printing by moving print media in a given direction (sub-scanning direction) while moving a print head in a direction (main scanning direction) crossing the sub-scanning direction. The serial type printing apparatus moves a relatively short print head over stationary print media in the main scanning direction to print an image of a width corresponding to that of the print head. Once the single main scan is finished, the serial type printing apparatus conveys the print media by a predetermined amount. This operation is repeated to form an image all over the print media.
A full line type printing apparatus adopts another form using a print head. The full line type printing apparatus uses an elongate print head consisting of a large number of ink jet print elements, ejection openings, and liquid paths that are in communication with the ejection openings (these are hereinafter collectively referred to as nozzles). The full line type printing apparatus uses the elongate print head (hereinafter also referred to as a full line head) fixed to the apparatus main body to carry out printing by continuously conveying print media in a direction crossing a longitudinal direction of the print head. This allows one line of image to be printed at a time during one scan operation, thus enabling an image to be quickly formed all over the print media.
Of these printing apparatus using a print head, the ink jet type (ink jet printing apparatus) that carries out printing by ejecting ink from the print head has various advantages as described below. The ink jet printing apparatus facilitates a reduction in the size of the print head, enables high-resolution images to be quickly formed, and requires reduced running costs because of its ability to achieve printing without the need for special processing of ordinary paper. The ink jet printing apparatus also makes reduced noise owing to the use of a non-impact strategy and enables color images to be easily formed using multiple color inks.
In particular, the full line type printing apparatus can further increase image forming speed as previously described because of its ability to achieve a desired print width during one printing operation (hereinafter also referred to as one-pass printing). The full-line type printing apparatus is thus expected to be used for on-demand printing, which is increasingly needed.
On-demand printing does not require several million sheets to be printed during one process as in the case of conventional newspapers and magazines; the printing speed required for on-demand printing is about 100 thousand sheets per hour. However, the manual operation required for on-demand printing needs to be reduced. In this regard, the full-line type printing apparatus is advantageous; in spite of its printing speed lower than that of conventional offset printing apparatuses, the full-line type printing apparatus enables the required manual operation to be reduced because it eliminates the need to produce a printing plate and also enables small amounts of many types of printed matter to be obtained both easily and quickly. Owing to these advantages, the full-line type printing apparatus is optimum for on-demand printing.
The full-line type printing apparatus used for on-demand printing needs to achieve a printing quality typified by a high resolution of 600×600 dpi (dots/inch) for monochromatic print documents such as texts or 1,200×1,200 dpi for full color images such as photographs. The required printing speed is at least 30 pages of A3-size print media per minute.
Moreover, on-demand printing very often involves the printing of print media of several sizes; an image taken using a digital camera or the like may be printed on an L-sized sheet as in the case of conventional silver photographs or on small media such as a postcard.
However, for full-line type print heads, particularly those which enable photographic images to be printed on large-sized sheets, it is very difficult to process the ejection openings and ink jet print elements provided all over the width of a print area without causing any defects. For example, the print head requires about 14,000 ejection openings (print width: about 280 mm) to achieve printing on A3-sized sheets at a density of 1,200 dpi. It is very difficult to process all of the large number of ejection openings and the corresponding ink jet print elements during a manufacture process without causing any defects. If such print heads were successfully manufactured, efficiency percentage would be low and enormous manufacture costs would be required.
Thus, the use of what is called a connecting head H shown in FIGS. 17 and 18 has been proposed for the full-line type ink jet printing apparatus using an elongate print head. The connecting head H is an elongate print head in which a plurality of relatively inexpensive, short chips CH used in serial type ink jet printing apparatuses are precisely arranged. To form a color image using such a connecting head H, a plurality of (in the figure, four) connecting heads H1 to H4 shown in FIG. 19 are arranged in association with a plurality of inks, a cyan (C), magenta (M), yellow (Y), and black (Bk) inks.
As a full-line type print head that can eject four color inks from the same chip, a connecting head has been proposed in which such chips are staggered as shown in FIG. 20. The connecting head shown in FIG. 20 can advantageously have a smaller dimension than the print head configured as shown in FIG. 19, in a direction orthogonal to the direction in which the print heads are arranged.
In each of the print heads H shown in FIGS. 19 and 20, the chips CH are connected together so that their connecting portions are overlapped in the arranging direction. In the connecting head, inks are alternately ejected from the nozzles to land on the print media at the same position. Alternatively, ink ejections from the nozzles are controlled in accordance with a predetermined operating ratio of the nozzles in the connecting portions so that the density of the images printed by the connecting portions is the same as the density of the images printed by the nozzles in non-connecting portions of the connecting head. However, the differences of the characteristics of the nozzles (e.g. landing deviation) tend to cause the image degradation because the image is printed by different nozzles. The connecting portions for the respective colors are also present at the same location in the nozzle arranging direction. Thus, when a color image is formed on print media, inks ejected from the connecting portions for a plurality of colors overlap at the same position on print media. Thus, the degradation of the images printed by the connecting portions is more noticeable than images printed by the non-connecting portions. This may cause a stripe-like density unevenness (connecting stripes). The connecting stripes may degrade image quality.
Thus, for the connecting head configured as shown in FIG. 19, for example, Japanese Patent Application Laid-Open Nos. 5-238003 and 8-25635 disclose a method of avoiding the overlapping of the connecting portions of the respective color print heads as shown in FIG. 21.
For the connecting head in which a plurality of color nozzles are arranged in one chip CH as shown in FIG. 20, a method has also been proposed which avoids the overlapping of the connecting portions of the nozzle array for the same color as shown in FIG. 22 (see Japanese Patent Application Laid-Open No. 2000-289233)
In the print head configured as shown in FIG. 22, the color inks from the connecting portions in the same chip do not overlap. This is expected to make unnoticeable the density unevenness appearing like dense stripes. However, this configuration may cause stripe-like density unevenness (connecting stripes) according to another cause. For example, Japanese Patent Application Laid-Open No. 2002-67320 describes that if the print head shown in FIG. 22 is used to print an image of a high print duty at a high speed, the nozzles in the connecting portions may cause end deviation, resulting in white stripes at the connecting portions. The end deviation is a phenomenon in which an inward shift within the nozzle array occurs in the position where an ink droplet ejected from a nozzle located near an end of the nozzle array lands on the print media. As a measure for solving this problem, Japanese Patent Application Publication No. 02980429 discloses a print head shown in FIG. 23.
In the nozzle shown in FIG. 23, in each of the connecting portions of chips CH, at least one nozzle in one of the chips overlaps at least one nozzle in the other chip. The overlapping nozzles print the same raster. This reduces the print duty of each nozzle to half, thus suppressing the end deviation.
A print head shown in FIG. 8 has also been proposed. The print head includes a plurality of (in the figure, two) connecting heads H1 and H2 that eject the same color ink. In this print head, the connecting portions of the chips CH in the connecting head H1 are displaced from the corresponding connecting portions of the chips CH in the connecting head H2 in the nozzle arranging direction. Such a print head enables a pixel to be printed via a nozzle in one of the connecting heads, which nozzle may cause end deviation, to receive an ink droplet ejected from a normal nozzle in the other connecting head, which nozzle does not cause end deviation. Such a nozzle arrangement has also been disclosed in Japanese Patent Application Laid-Open Nos. 5-238003 and 8-25635.
The printing methods disclosed in Japanese Patent Application Laid-Open Nos. 5-238003 and 8-25635 are effective in visually reducing stripes caused by end deviation if printing is carried out under given printing conditions. However, these printing methods may be ineffective if the amount of end deviation varies as a result of a variation in printing conditions.